Non Booting Disk

Please does anyone know a fix for drives not booting as startup Disks on the system, instead it shows me the question mark folder sign, except when used in target disk mode it boots and performs well with all applications and files intact. its a macbook pro 13.3", Yosemite 10.10.5 OS X. I have tried another disk with Mountain Lion installed and it works on the system. Any help please?